burnt valve
Racer
Joined: Dec 2006
Posts: 417
Likes: 40
From: Swansea IL
Testing the Cylinder
Put the piston in the cylinder you which to test at TDC. Put compressed air in the cylinder (leak down tester) and then listen in the intake and then the exhaust.
A compression gauge will only tell you that the cylinder isn't up to spec. It won't tell you where the problem is.
Both valves should be closed and sealing. Check where you hear the air leaking and you have your culpret.
RGDS
Bob
A compression gauge will only tell you that the cylinder isn't up to spec. It won't tell you where the problem is.
Both valves should be closed and sealing. Check where you hear the air leaking and you have your culpret.
RGDS
Bob
Le Mans Master
Joined: Jan 2003
Posts: 6,575
Likes: 1
From: Tuttle OK
Cruise-In VIII Veteran
it can vary - low power, missing, kinda chugging.
i've done compression tests as well as bob's method - both will give a clue.
using bobs method, if it blows out the exhaust or intake, check for a busted spring as well.
i've done compression tests as well as bob's method - both will give a clue.
using bobs method, if it blows out the exhaust or intake, check for a busted spring as well.
Last edited by Avette4me; Feb 17, 2007 at 08:33 AM.
Racer
Joined: Dec 2006
Posts: 417
Likes: 40
From: Swansea IL
Symptoms
If the valve is burnt to the extent that it is effecting performance here are my thought.
First - the cylinder with the burnt valve will have less/little compression compared to the others.
Second - When idling, I would assume a miss or at least rough running at low rpm. When you pick up the rpm the roughness will lessen as the other cylinders are masking the bad one.
You could remove spark plug wires one at a time while idling and when you don't see much rpm drop, that is a good candidate for a bad cylinder.
Checking under the valve covers for a broken spring is a good idea. That will keep the valve from seating. Be careful if that is the problem. Without proper spring pressure the keepers come undone and the valve drops into the cylinder. Not something you want to happen.
RGDS
Bob
First - the cylinder with the burnt valve will have less/little compression compared to the others.
Second - When idling, I would assume a miss or at least rough running at low rpm. When you pick up the rpm the roughness will lessen as the other cylinders are masking the bad one.
You could remove spark plug wires one at a time while idling and when you don't see much rpm drop, that is a good candidate for a bad cylinder.
Checking under the valve covers for a broken spring is a good idea. That will keep the valve from seating. Be careful if that is the problem. Without proper spring pressure the keepers come undone and the valve drops into the cylinder. Not something you want to happen.
RGDS
Bob
Thread Starter
Burning Brakes
Joined: Apr 2005
Posts: 1,227
Likes: 0
From: Atlanta Ga
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
Don't jump out and hit the panic button. Start with the simple stuff. Take a spray bottle full of water and wet down the plug wires with it running. Let it dry and then pull the plug wires 1 at a time with it running. The weak cylinder won't cause as much change-You're looking for any changes in the way it's running. Then pull the plug from that cylinder and LOOK at it. Look for gap, fouling, cracks, then look at the wire. Check it with an ohmmeter. Pull another plug to compare. Swap a plug wire from another cylinder.If you can't pin it down, then do a compression test- difference from hi to low should be no more than 10% (general guideline here), then a leakdown test. Unless there's a bunch of backfiring or popping thru the exhaust going on, I'd be willing to bet it's an ignition problem.
BTW: be careful when pulling plug wires when the engine is running- them sparks HURT!
BTW: be careful when pulling plug wires when the engine is running- them sparks HURT!
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